In a typical ink jet printer using a multi-nozzle head, data as to each of four colors (i.e., red, green, blue and black) regarding an input image are processed in a manner so that the multi-nozzle head forms a printed color output image on a recorder medium, which may be a suitable paper or transparency.
However, ink jet printers may produce non-uniform print density with respect to the image formed on the recorder medium. Such non-uniform print density may be visible as so-called "banding". Banding is evinced, for example, by repeated variations in the print density caused by delineations in individual dot rows comprising the output image. Thus, banding can appear as light or dark streaks or lines within a printed area. One factor causing banding is unintended variation in ink droplet volume. Unintended variation in ink droplet volume in turn may be caused by electrical resistance variation of a plurality of heaters in communication with the ink droplet, nozzle diameter variation, and/or the presence of damaged nozzles. Therefore, a problem in the art is non-uniform print density due to variation in nozzle physical attributes which in turn leads to variation in ink droplet volume.
Moreover, the ability of some prior art ink jet printers to produce halftone images has been limited because the ink jet print heads belonging to such printers produce ink droplets having a fixed volume. Marks produced by such droplets are of a fixed size and the same intensity. Consequently, these ink jet print heads utilize spot density, rather than spot size, to produce a gray-scale image. That is, these ink jet print heads produce various shades of gray by varying the density of the fixed size ink marks such that darker shades are produced by increasing spot density and lighter shades are produced by reducing spot density. However, such printers have reduced spatial resolution, thereby limiting the ability of the ink jet printer to produce finely detailed images. Spatial resolution is reduced because varying frequency of the constant spot size in a printed area obtains lower resolution when compared to keeping a constant frequency but varying the spot size. Moreover, directing multiple droplets at a single location of the recorder medium to increase spot size tends to reduce the operating speed of the printer to an unacceptably low level and may even produce elongated or elliptical dot patterns. Therefore, another problem in the art is difficulty producing ink droplets that vary in size.
An ink jet printer device directed to controlling ink droplet volume and gray-scaling is disclosed in U.S. Pat. No. 4,563,689 titled "Method For Ink-Jet Recording And Apparatus Therefor". This patent discloses an ink jet recording apparatus and process in which the droplet size is controlled to obtain halftone-graduation recording. According to this patent, a preceding pulse is applied to an electromechanical transducer prior to applying a main pulse so as to control the position of the ink meniscus in the nozzle and thereby control droplet size.
However, this patent requires use of an electromechanical transducer to control ink droplet size. Use of an electromechanical transducer is not preferred because electromechanical transducers are difficult and costly to fabricate due to their structural complexity.
Another type of ink jet printer which addresses the afore mentioned problems of controlling ink drop volume and gray-scaling is disclosed in U.S. patent application Ser. No. 08/783,256 titled "Ink Jet Printhead For Multi-Level Printing". In this device, a recorder medium is reciprocatingly moved adjacent a plurality of nozzles in order to sequentially apply four colors of an input image onto the recorder medium. To achieve this result, an ink droplet in each nozzle is under a predetermined static back-pressure in order to propel the ink droplet toward the recorder medium. However, before the ink droplet is propelled toward the recorder medium, it is initially restrained or held in the orifice by surface tension even though the ink droplet is under static back-pressure. This results in an ink meniscus bulging outwardly at the nozzle orifice without leaving the orifice. This is so because, by design, the back-pressure is initially insufficient to overcome the ink droplet's surface tension. Therefore, in order to print on the recorder medium, the surface tension of the ink droplet is controllably decreased, so that the ink droplet is released from the nozzle orifice at the desired time and propelled onto the recorder medium by the previously mentioned static back-pressure. To decrease surface tension, a voltage pulse is applied to an electrical resistance heater that is in heat transfer communication with the ink droplet. Heating of the resistance heater by the voltage pulse heats the ink droplet, thereby reducing the surface tension of the ink droplet. Of course, the static back-pressure acting on the ink droplet coacts with the simultaneous decrease in surface tension to eject the ink droplet from the orifice and propel it toward the recorder medium. Means are provided to obtain uniform print density by controlling the heat energy supplied to the ink droplet. However, potential for heating of the ink in this type of ink jet printer can at least theoretically, lead to boiling and void formation in the ink. Void formation is the formation of bubbles (i.e., voids) in the ink. Void formation is undesirable because the bubbles resulting from void formation could coalesce and block the nozzle orifice. Blocking the nozzle orifice interferes with proper ejection of the ink from the nozzle, thus leading to undesirable printing defects in the output image. Although this printer addresses the problem of banding, it does not expressly address the potential for void formation. Therefore, yet another problem in the art is the potential for void formation caused by excessive heating of the ink.
Therefore, what has long been needed is an imaging apparatus and method adapted to control ink droplet volume, so that printing of anomalous non-uniformities, such as "banding", are avoided and so that print density can be controllably varied to provide gray-scaling at each dot or pixel and so that the potential for void formation in the ink is reduced.